left 4 dead 2 was not good, so left 4 dead 3 will not come.
i believe there will be a new engine. because the source engine is kinda outdated. on the other hand, ep3 was supposed to be announced at E3 last year, but was pulled due to fine tuning. half life 3? no ep3 has to come first, it was garrunteed a trilogy when ep1 was released. i believe a new engine will be released, maybe even with portal 2, but most likely with a game in the future